An internal combustion engine requires that all of its pistons have substantially the same weight. That is to say, that the variation in weight from one piston to the next is less than 0.8%. Hence, for example, engines having pistons weighing about 500 grams will typically require that there be no more than 4 grams weight difference from one piston to the next.
Owing in part to significant variations in the as-cast weight of pistons leaving conventional piston molds, it has heretofore been common practice in the industry to cast extra metal onto the piston in a region thereof known as a "balance pad". The balance pad often contains as much as 20-25 grams of extra metal much of which is subsequently machined away in a weight-control station located downstream of the casting operation where the weight of the piston is brought within acceptable limits. The weight-control operation involves weighing of the piston before removing metal from the balance pad, machining metal from the balance pad and then re-weighing the piston to check its final weight. This weight control procedure is time consuming and costly and, if eliminated, would improve the productivity of the piston manufacturing operation and reduce the cost of pistons produced thereby.
It would be desirable to eliminate the machining for weight control step and shift the weight control to the casting station if possible. Heretofore that has not been possible. One of the reasons for significant as-cast weight variations is the design of the mold used to shape the piston. Heretofore the mold design, the tolerances between the several movable mold components, and the stack up of those components has allowed for wide weight variations from one piston to the next all cast from the same mold. In this regard heretofore, the core forming the firing face of the piston (hereafter dome) typically located against a surface or surfaces on the mold segments which shape the sidewalls of the piston, which, in turn, is located against a stationary mold part. Since the several mold parts each has its own manufacturing tolerance and allowances for clearances within the mold, the stack up of the mold components from one casting to the next allowed for wide variations of spacing between the core used to shape the piston's firing face and the stationary base of the mold. This type of location system simply does not control the location of the dome-forming core accurately enough to control piston weight. Some clearances must be provided between the several components of the mold and these clearances reveal themselves as locational error of the dome core relative to the piston interior as the molds' sidewall segments randomly move up and down in this clearance. Hence, each time the several mold components came together to form the mold cavity, the components are positioned in a slightly different position from the previous casting. This particularly affects the location of the dome-forming core relative to the stationary base of the mold.
It is an object of the present invention to provide a permanent mold for the gravity casting of internal combustion engine pistons which mold (1) produces a series of cast pistons having little weight variation from one piston to the next, (2) can be readily converted to cast a different series of pistons having a different weight than the previous series of pistons, and (3) obviates the need for a downstream station for machining the piston to adjust its weight. It is a further object of the present invention to provide a permanent mold for the gravity casting of pistons wherein spacer means are provided between a dome-forming core and a stationary base component of the mold to establish a fixed distance therebetween from one casting to the next and thereby insure that all of the pistons cast from that mold using the spacer have substantially the same weight. This and other objects and advantages of the present invention will become more readily apparent from the detailed description thereof which follows.